1. Field of the Invention
This invention is directed to an improved technique for coating substrates using powdered coating materials. The technique is particularly useful for coating substrates which are heat sensitive, such as cellulosic or plastic substrates. In preferred embodiments, it enables the use of powdered coating materials in manufacturing processes, such as the membrane press coating process and roll coating processes, wherein such powder coating materials have not been successfully used in the past. The invention is particularly useful for the production of coated wood articles, such as medium density fiberboard and particle board panels.
2. Description of Related Art
The application of dry coating materials on manufactured articles has become increasingly important because of their significant environmental advantage over the use of liquid coating materials, such as paints. These advantages principally involve avoiding, or minimizing, the use of volatile organic solvents, and thereby avoiding the air pollution and health concerns associated with such solvents.
Dry coating materials have generally been applied as powders or as films. Dry powder coating methods have involved depositing a dry, free flowing powder on a substrate and then heating the powder to cause it to fuse and cure. Since the heating step has generally required exposing the substrate to temperatures which cause deterioration of heat sensitive materials, such as those based on wood and/or plastic materials, the use of such dry powder coating methods has been primarily directed to coating metal articles. Recently, dry coating powder materials which are capable of fusion and curing at temperatures consistent with their use on wood based substrates have been introduced. Examples of such lower temperature coating materials are described in commonly assigned U.S. Pat. Nos. 5,714,206 and 5,721,052 and in U.S. patent applications Ser. Nos. 09/191,398 [3477-05-99] and 09/316,545. The commonly owned patents and applications referred to throughout this application are hereby incorporated by reference in this application. While these dry coating methods and materials have produced excellent textured coatings on wood based substrates, it has been difficult to produce smooth high gloss coatings with these methods and materials. Moreover, relatively thick coatings, approximately 5 mils thick, have been required to provide coatings with good moisture resistance and other barrier properties.
Membrane pressing is an important commercial process for laminating sheets on composite wood panels, such as medium density fiberboard (MDF) panels. The process involves vacuum forming a thermoplastic sheet on a MDF profile/substrate and activating a preapplied glue to bind the sheet to the profile. The technique is generally limited commercially to laminating vinyl sheets on relatively smooth and flat profiles, or substrates. If the profile is irregular, having grooves or other surface effects, the laminated film tends to not be uniformly bound to the profile. If the profile is not finished to a suitable degree of smoothness, surface irregularities appear through the laminated film. Moreover, the laminated film may exhibit irregularities, such as bubbles or orange peel surface texture, caused by gases trapped, or released from volatile components, between the sheet and the profile. A further problem occurs when localized bonding defects result in delamination, or peeling, of the film from the profile.